High-speed welding is possible by mash seam welding in which two plate-like workpieces are continuously welded together at their overlapping edge portions while the edge portions are squeezed to be mashed by and between a pair of roller electrodes, with a welding current being applied between the pair of roller electrodes. Since the mash seam welding is suitable for mass production, it is widely employed in production lines for steel plates, automobiles, etc.
In the mash seam welding described above, the two plate-like workpieces are positioned relative to each other by a clamping or guiding device that predetermined portions of the workpieces overlap each other, and the workpieces are fed so that the overlapping portions are squeezed by the pair of roller electrodes. Then, the two plate-like workpieces are continuously welded with a welding current applied between the roller electrodes while the overlapping portions of the workpieces are firmly squeezed and mashed by the roller electrodes.
In such mash seam welding, a so-called "weld nugget" N is formed based on a welding current applied to the pair of roller electrodes, such that the weld nugget N is located in a central part of the thickness of a portion of two plate-like workpieces 12.sub.H, 12.sub.M squeezed by the pair of roller electrodes, as shown in FIG. 10. The formed weld nugget N extends across an interface G of the two plate-like workpieces 12.sub.H, 12.sub.M, so that the weld may be given a predetermined strength (tensile strength). In FIG. 10, W.sub.G represents a width dimension of the interface G, while W.sub.N represents a width dimension of the weld nugget N in the direction of the interface. The weld strength obtained by the mash seam welding depends upon a ratio W.sub.N /W.sub.G, namely, a nugget ratio R.sub.N, which is the ratio of the width dimension W.sub.N of the weld nugget N to the width dimension W.sub.G of the interface G. The weld strength of the mash seal welding increases as this nugget ratio R.sub.N increases.
Where two plate-like workpieces having different thickness values are welded according to a conventional mash seam welding process, the interface G of the two plate-like workpieces 12.sub.H, 12.sub.M mashed by the pair of roller electrodes is formed at a position which is offset from the center of the thickness of the mashed portion in the direction toward the thin plate-like workpiece 12.sub.M, as indicated in FIG. 11. However, the weld nugget N is formed at a position intermediate between the pair of roller electrodes, that is, at a central part of the thickness of the mashed portion of the two plate-like workpieces 12.sub.H, 12.sub.M squeezed between the pair of roller electrodes. Accordingly, it is difficult to form the weld nugget N in alignment with the interface G of the two plate-like workpieces 12.sub.H, 12.sub.M, and is therefore difficult to obtain the desired weld strength in some cases.
While it is of course possible to increase the nugget ratio R.sub.N indicated above by increasing the amount of the welding current to thereby form a large weld nugget N, an increase in the nugget ratio R.sub.N leads to easy occurrence of cracking on the surfaces of the plate-like workpieces due to enlargement of the weld nugget N which undergoes rapid volumetric expansion. This cracking may develop into rupture in a subsequent press-forming process, for example. In view of this, the application of the mash seam welding is conventionally limited to the workpieces whose thickness ratio is not larger than a predetermined upper limit, for example, about 1.5, and the mash seam welding is not conventionally practiced where the thickness ratio of the workpieces exceeds the upper limit.
On the other hand, there has been proposed a seam welding process in which a shim is superposed on a thinner one of two plate-like workpieces so that the interface G of the two plate-like workpieces is located intermediate between a pair of roller electrodes, as disclosed in JP-A-63-63575. This seam welding process requires the shim as well as the two plate-like workpieces to be clamped by a clamping device adapted to position the two plate-like workpieces relative to each other. Therefore, this process has a drawback that the operation efficiency is inevitably lowered. Further, where the two plate-like workpieces are positioned relative to each other by using guide rollers while the workpieces are fed toward the roller electrodes, there is a drawback that the above seam welding process using the shim cannot be practiced.
The present invention was made in the light of the above background situation. It is therefore an object of the present invention to provide a mash seam welding process and a mash seam welding apparatus, which permit a sufficient weld strength even where two plate-like workpieces have different thicknesses.